1. Field of The Invention
The present invention relates to a method for producing adiponitrile. More particularly, the present invention is concerned with an improved method for producing adiponitrile by the electrohydrodimerization of acrylonitrile in an undivided cell. The electrolyte used is composed of an acrylonitrile-containing emulsion comprised of an aqueous phase and an organic phase, wherein the aqueous phase contains a specific amount of an ethyltributylammonium salt. The method of the present invention is improved with respect to the prevention of the corrosion of the cathode metal or metal alloy employed in the cell.
2. Discussion of Related Art
Production of adiponitrile by electrohydrodimerization of acrylonitrile is known in the art. The reaction therefor is believed to proceed as follows. ##STR1## Besides the above predominant reactions, the following side reactions occur. EQU CH.sub.2 .dbd.CHCN+2H.sup.+ +2e.fwdarw.CH.sub.3 CH.sub.2 CN ##STR2##
Adiponitrile has been produced on a commercial scale by electrohydrodimerization of acrylonitrile in which a cell divided into two compartments by a membrane is employed. The membrane is employed in order to prevent the acrylonitrile from undergoing oxidation at the anode, which would lead to a decrease in the yield of adiponitrile produced. However, the electrohydrodimerization of acrylonitrile which employs a membrane has drawbacks in that the power consumption due to the membrane resistance as well as the cost of the membrane are high.
Accordingly, various methods of conducting electrohydrodimerization of acrylonitrile in an undivided cell, in which no membrane is used, have been proposed in the art.
For example, it has been proposed to conduct electrohydrodimerization of an olefinic compound such as acrylonitrile or the like by a method comprising electrolyzing an aqueous solution having dissolved therein at least about 0.1% by weight of the olefinic compound, quaternary ammonium ions in a concentration from about 10.sup.-5 to about 0.5 gram mol per liter and at least about 0.1% by weight of a phosphate, borate or carbonate of an alkali metal in an undivided cell having a cadmium cathode and a carbon steel anode (see the Examples of U.S. Pat. No. 3,897,318). This method is advantageous in that the cadmium cathode is resistant to corrosion, but is disadvantageous in that cadmium used as the cathode has a high toxicity and therefore, a special treatment of waste water and other costly, time-consuming operations are necessary.
In conducting the electrohydrodimerization of acrylonitrile in an undivided cell, it is preferred that the cathode of the cell be comprised of a metal exhibiting a high hydrogen overvoltage. It is known that besides the above-mentioned cadmium, mercury and lead exhibit a high hydrogen overvoltage. Lead, which is less toxic as compared with cadmium and mercury, is used as a cathode material in an undivided cell for the electrohydrodimerization of acrylonitrile. For example, U.S. Pat. No. 3,898,140 and U.S. Pat. No. 3,689,382 disclose the electrohydrodimerization of acrylonitrile in an undivided cell in which lead has been used as the cathode material and a combination of an alkali metal salt and an ethyltributylammonium salt has been used as the electrolysis supporting salt. In these U.S. patents, the purposes of using the ethyltributylammonium salt are solely to increase the conductivity of the electrolyte and hence the yield of adiponitrile, and accordingly, the ethyltributylammonium salt concentration of the aqueous phase is generally not greater than 0.01 mol/liter. However, the methods as disclosed in these U.S. patents have a drawback in that the corrosion of the cathode is rapid. Further, the method of U.S. Pat. No. 3,898,140 is accompanied by a drawback in that the evolution of hydrogen gas is still intense, which evolution is undesirable from the viewpoint of adiponitrile yield.
To overcome the problem of the evolution of hydrogen gas at the cathode in a process for producing adiponitrile which comprises electrolyzing an emulsion containing acrylonitrile and, as electrolysis supporting salt, a combination of an alkali metal salt and a quaternary ammonium salt in an undivided cell having a lead alloy cathode, it has been proposed in Japanese Patent Application Publication Specification No. 61-21316/1986 to continuously or intermittently take the electrolyte out of the electrolytic cell and recycle the same through a column packed with a chelate resin to the electrolytic cell. It is noted that in the Examples of this publication, use is made of a single undivided cell, not connected to any other cells, which is provided with lead alloy cathode having a rectangular current-passing surface of only 90 cm in length and an anode also having a rectangular current-passing surface of the same size.
In the production of adiponitrile on a commercial scale, it is necessary to pass a large amount of electric current between the anode and the cathode. Accordingly, the anode and the cathode should have a large current-passing surface. The area of a generally employed rectangular current-passing surface of the anode or cathode can be increased either by increasing the length of the surface, along which the emulsion flows, or increasing the width of the surface, which is perpendicular to the direction of flow of the emulsion. Generally, increasing the length of the current-passing surface is preferred to increasing the width of the surface from the viewpoint of the cost of pumps, piping and other facilities for circulating the emulsion. with the increase of the length of the current-passing surface, the amount of oxygen gas evolved is incresed at the terminus of the current-passing surface, which terminus generally corresponds to the outlet of the cell and so is hereinafter referred to as the outlet of the cell. In an example of Japanese Patent Application Publication Specification No. 61-21316/1986, the use of an ethytributylammonium salt in an amount of 0.009 mol/l is indicated. However, the electrolysis of this example is accoompanied by rapid corrosion of the cathode, when the evolution of oxygen is intense at the outlet of the cell. In this publication, a tetraethylammonium salt is employed at a relatively high concentration. However, as demonstrated in a comparative example given later, the use of the tetraethylammonium salt even at a relatively high concentration is not effective for retarding the corrosion of the cathode where the evolution of oxygen is intense at the outlet of the cell. Therefore, the method as disclosed in this publication is not advantageous from the viewpoint of cathode corrosion.
As is apparent from the foregoing, the hitherto known methods have drawbacks, and hence there is still a strong demand in the art for an effective method of producing adiponitrile by electrohydrodimerization of acrylonitrile in an undivided cell provided with a lead or lead alloy cathode, which method is free from or remarkably improved in respect of the problems such as corrosion of the cathode.